Simple program to encrypt and decrypt images, through image manipulation. Seems secure enough.

crypt.py

from PIL import Image
import sys
import hashlib
from getpass import getpass
import random

swap = ((1, 2, 3, 0), (3, 2, 1, 0), (1, 3, 0, 2), (0, 3, 1, 2), (1, 2, 0, 3), (2, 0, 3, 1), (2, 1, 0, 3), (3, 1, 2, 0), (0, 1, 3, 2), (2, 3, 1, 0), (0, 3, 2, 1), (3, 1, 0, 2), (3, 0, 2, 1), (3, 2, 0, 1), (2, 1, 3, 0), (0, 2, 3, 1), (3, 0, 1, 2), (2, 3, 0, 1), (0, 2, 1, 3), (1, 3, 2, 0), (2, 0, 1, 3), (0, 1, 2, 3), (1, 0, 2, 3), (1, 0, 3, 2))

def encrypt(file, pword):
 img = Image.open(file)
 img = img.convert('RGBA')
 pixels = img.load()
 width, height = img.size
 i_cnt, j_cnt = 0, 0
 hashed = hashlib.md5(pword.encode()).hexdigest()
 random.seed(hashed)
 for ii in range(width):
  for jj in range(height):
   cur = list(pixels[ii, jj])
   for kk in range(4):
    cur[kk] = cur[kk]^((ord(hashed[j_cnt]) + int(random.random() * 255)) % 256)
    j_cnt += 1
    j_cnt = j_cnt % len(hashed)
   num = (ord(pword[i_cnt]) + int(random.random()*24)) % 24
   cur = [cur[swap[num][0]], cur[swap[num][1]], cur[swap[num][2]], cur[swap[num][3]]]
   
   i_cnt += 1
   i_cnt = i_cnt % len(pword)
   pixels[ii, jj] = tuple(cur)
 img.save('encrypted-' + file)

def decrypt(file, pword):
 img = Image.open(file)
 pixels = img.load()
 width, height = img.size
 i_cnt, j_cnt = 0, 0
 hashed = hashlib.md5(pword.encode()).hexdigest()
 random.seed(hashed)
 for ii in range(width):
  for jj in range(height):
   cur = list(pixels[ii, jj])
   rr = [random.random(), random.random(), random.random(), random.random()]
   r2 = random.random()
   num = (ord(pword[i_cnt]) + int(r2*24))%24
   cur[swap[num][0]], cur[swap[num][1]], cur[swap[num][2]], cur[swap[num][3]] = cur
   for kk in range(4):
    cur[kk] = cur[kk]^((ord(hashed[j_cnt]) + int(rr[kk] * 255)) % 256)
    j_cnt += 1
    j_cnt = j_cnt % len(hashed)
   i_cnt += 1
   i_cnt = i_cnt % len(pword)
   pixels[ii, jj] = tuple(cur)
 img.save('decrypted-'+file)

dec = False
infile = ""
if not len(sys.argv) > 1:
 infile = input("File: ")
else:
 import argparse
 parser = argparse.ArgumentParser()
 paa = parser.add_argument

 paa('input_file')
 paa('-d', action='store_true', dest='decrypt')
 res = parser.parse_args()
 
 dec = res.decrypt
 infile = res.input_file
password = getpass()

if dec:
 decrypt(infile, password)
else:
 encrypt(infile, password)

big nerd ❤️

Small nitpick but using python's random library is not secure. You should be using the secrets module. (See: warning in the random library doc)

Ohhh thanks, I'll rewrite it in a bit

Nevermind, the correctness of your system relies on the ability to seed your PRNG. You can't do that with secrets since it reads from /dev/random. So it looks like you'll have to use the random library.

Guess I'm sticking to it haha

Just made it non-destructive on encryption, decryption was already fine. This way if you do the wrong operation, you can basically just undo it by deleting the new file